Prime Minister David Cameron will warn today (16 April) that the Scottish National Party (SNP) will act as "the chain to Labour's wrecking ball" and create a "coalition of chaos" if Ed Miliband does a deal with Nicola Sturgeon after the election.
Cameron will use a speech in Scotland to warn that a Labour government propped up by the SNP will mean "job losses, massive tax rises and an economy back on the brink of bankruptcy".
His comments come as a new analysis disclosed that Ed Miliband voted in the same way as SNP MPs more than 90% of the time in the past five years, including 27 out of 28 votes on welfare and 62 out of 65 votes on the economy.
He will make his remarks during the unveiling of the Scottish Conservative manifesto in Glasgow.
"We meet here in Scotland in the middle of a massive political fight. We've got Labour and the SNP on opposite sides – slugging it out – but if you take a step back they're really on the same side," Mr Cameron will say.
"You have a weak Labour party, who want more spending, more borrowing, more debt and more taxes. And the people who will prop them up, the SNP – who want even more spending, more borrowing, more debt and more taxes."
Despite denials by Labour leader Ed Miliband that he would be willing to do a deal with the nationalists, the Tories believe that their warnings about Labour being "held to ransom" by SNP leader Nicola Sturgeon and Alex Salmond, the former SNP leader, are having an increasing effect on voters.
He will warn that a Labour government reliant on the SNP would "pose a clear threat" to the future of the UK.
The Prime Minister will add: "Together, they pose a clear threat to the future of our United Kingdom. A coalition of chaos.
"The SNP acting as the chain to Labour's wrecking ball, running right through our economic recovery - and it will be you who pays the price. With job losses, massive tax rises and an economy back on the brink of bankruptcy."
Meanwhile, United Kingdom Independence Party (Ukip) leader Nigel Farage has said he is holding informal talks with the Conservatives ahead of the election, as he hinted that he would be prepared to support a minority Tory government led by David Cameron.
The UK Independence Party leader was asked at his party's manifesto launch if it would be a "red line of yours if the Conservatives get rid of David Cameron before you would be willing to support a Conservative Government".
The Ukip leader joked that "if I was a Conservative I would want to get rid of David Cameron".
Asked whether he had been involved in informal discussions about potential post-election arrangements with Conservatives, Farage said: "If I meet people in a social environment, I'm a gregarious cove and I generally speak to them. We have discussions with people who are in all of the other political parties, yes, of course that goes on."